1. Field of the Invention
This invention relates to technology for converting tone image data into dot data represented by dot on/off state, and in particular relates to technology for processing a predetermined number of neighboring pixels as a single unit in order to speed up conversion of image data, while avoiding worsening of image quality.
2. Description of the Related Art
Image display devices that represent images by forming dots on a display medium, namely a printing medium or liquid crystal screen, are widely used as output devices of various kinds of image machines. While these image display devices can locally only represent a state of either forming a dot or not, by unit of appropriately controlling dot formation density depending on tone values of an image, it becomes possible to represent an image with continuously varying tone.
In order that dots are formed with appropriate density depending on tone values of an image in these image display devices, as the method for determining the dot on/off state for each pixel, a method termed the error diffusion method is widely used. In the error diffusion method, error in tone representation created by forming a dot or not forming a dot on a pixel of interest is diffused into undetermined pixels neighboring the pixel of note and stored in memory, and when determining dot on/off state for undetermined pixels, determining the dot on/off state so as to cancel out error diffused from neighboring pixels. By using the error diffusion method, dot on/off state can be determined in such a way as to cancel out error of tone representation created by neighboring pixels, so that it is possible to determine dot on/off state such that density is appropriate depending on tone values of the image.
By using the error diffusion method, while it is possible to display an image of high quality by unit of forming dots at appropriate density depending on the image, on the other hand, tone error must be diffused into neighboring pixels each time that dot on/off state is determined, and if the number of pixels making up an image is large, the process is time consuming and it becomes difficult to represent the image rapidly. To solve such problems, the inventors of the present application have developed technology for treating a predetermined number of neighboring pixels as a unit, and determining dot on/off state while diffusing error on a per-unit basis (Japanese Patent Application 2001-185789A).
However, where determination of dot on/off state is carried out on a per-unit basis, there were instances in which image quality was not necessarily good. For example, where determination of dot on/off state is carried out on a per-unit basis, a similar process is repeated for each unit, and over several units, dots are formed on pixels at the same locations in the unit, this being visible as a cyclical pattern, which poses a risk of degrading image quality. Also, where determination of dot on/off state is carried out on a per-unit basis, the problem of a concern that a cyclical pattern will occur is not limited to the error diffusion method, but can occur similarly in cases where other known methods are used.
Alternatively, where dots are not formed evenly on pixels in a unit, but formed with bias to specific pixel locations, while it may not go as far as occurrence of a cyclical pattern, by the fact of, for example, the printing pass that forms the dots being biased to a particular pass so that color irregularity occurs, image quality may become degraded in some instances. Further, where dot on/off state is determined on a per-unit basis, the problem of a concern that image quality will be degraded is not limited to the error diffusion method, but can occur similarly in cases where other known methods are used.